The present invention relates generally to voltage controlled oscillators (VCOs) for phase lock loop frequency synthesizers, and in particular to calibrating and tuning a VCO, and including operating point control for the VCO.
Conventional phase-locked loops (PLLs) are used in prior art circuits to synthesize local oscillator frequencies used in radio receivers. The most common type of PLL uses a VCO that depends on a varactor for tuning. But inexpensive varactor diodes cannot be used in PLLs that need to be tuned over very large frequency ranges.
A typical prior-art VCO is shown in FIG. 1 in a phase locked loop configuration and given reference numeral 100. The VCO constant denoted Kv is the slope of the frequency vs. control voltage curve, and when an attempt is made to tune a varactor controlled VCO, the required VCO constant is too large or the capacitance variation required becomes unachievable and the PLL has trouble locking. Such circuits, when they do lock, are very sensitive to digital noise, because a relatively small noise voltage is translated into a relatively large frequency perturbation. For example, in a 1.8-volt system needing a one-gigaHertz tuning range, a VCO constant of 1 GHz/volt would be far too large to be practical for use in a PLL with a loop bandwidth consistent with practical 802.11a radio operation.
In general, a smaller VCO constant will result in reduced phase noise. To keep the VCO constant relatively low, yet achieve a large tuning range, one prior art tuning methods includes switching in and out fixed capacitors and using the varactors to tune between the frequencies of only the fixed capacitors. A one gigaHertz tuning range, for example, would be implemented in ten 100 MHz subranges, with the VCO constant of 100 MHz/volt, achieving a twenty dB reduction.
Unfortunately, with mass-produced semiconductor devices such switched fixed capacitors vary with manufacturing process spread and with operating temperatures. So a calibration method and circuit are needed that can reduce the frequency uncertainties that would otherwise be introduced into PLL and VCO applications. Better yet, a built-in calibration method could help in obtaining a longer, more reliable product life.
It is desired to have an inexpensive VCO for a mass produced device, for example using CMOS technology. It is very difficult to make good quality VCOs in CMOS; the inductor Q""s, for example, are very poor. Therefore it is important that all other factors that contribute to phase noise are significantly reduced, the VCO constant being one of them. The VCO constant affects the loop dynamics of a PLL that uses the VCO, such as the phase margin and loop bandwidth of the PLL.
One desirable property of a VCO is that the VCO constant not only be kept reasonably low, but also stable and known.
FIG. 2 shows the frequency vs. control voltage curve for a fictional xe2x80x9cidealxe2x80x9d VCO. The curve is a straight line over a large frequency range, so that Kv is constant. FIG. 2 also shows what a frequency vs. control voltage curve looks like for a fictional prior-art CMOS VCO with no operating point control. The curve is not linear, but xe2x80x9cS-shaped.xe2x80x9d The slope of the curve varies with frequency, and such a variation is shown, by way of example, in FIG. 3. Furthermore, the frequency vs. control voltage curve itself varies with temperature, leading to drift in the free running frequency. Furthermore, the frequency vs. control voltage also varies from device to device, particularly when using inexpensive technology such as CMOS. Because of these and other effects, one cannot predict where the operating point, i.e., the point of the lock up of the PLL, will be.
Thus there is a need to provide a control mechanism to ensure that the operating point of the VCO is close to a desirable operating point.
An object of the present invention is to provide a voltage controlled oscillator with a relatively large frequency swing having an output in the gigaHertz range.
Another object of the present invention is to provide a calibration method for a VCO in a mass-produced semiconductor device, e.g., a CMOS device.
Another object of the present invention is to provide a method and an apparatus for controlling the operating point of a VCO used in a PLL frequency synthesizer.
A further object of the present invention is to provide a PLL frequency synthesizer circuit that includes a control mechanism to control the operating point of the VCO.
A further object of the present invention is to provide a wireless radio oscillator circuit.
Briefly, a frequency synthesizer calibrator embodiment of the present invention comprises a main charge pump that drives a voltage-controlled oscillator (VCO) through a loop filter. A second, replica charge pump can also drive the VCO, but is set up to output only its most positive or most negative analog output control voltage. Since the construction and characteristics of the replica charge pump duplicate the main charge pump, the main charge pump""s minimum and maximum analog control outputs can be cloned out to the VCO on demand. A VCO calibration procedure therefore includes switching the VCO to each of its ranges set by a bank of fixed capacitors, and using the replica charge pump to drive the VCO to its minimum and maximum frequency for each range setting. The min-max frequency data is stored in a lookup table, and operational requests to switch to a new channel frequency can be supported with a priori information about which fixed-capacitor range selection will be best.
Furthermore, one embodiments includes a sensor to sense the operating point of a VCO in a PLL configuration by sensing the input control voltage, and a controller that provides a switching input to a switchable bank of capacitors to change the operating point such the at VCO is maintained close to a desired operating point. The change can be determined according to calibration data.